1. Field of the Invention
The embodiments relate to booklet conveying devices, image forming systems, and booklet conveying methods. More particularly, the invention relates to a booklet conveying device that conveys a booklet formed by folding sheets of a recording medium (hereinafter, abbreviated to “sheets”) such as ordinary paper, recording paper, or transfer paper, an image forming system including the booklet conveying device, and a booklet conveying method to be performed by the booklet conveying device.
2. Description of the Related Art
Sheet processing apparatuses arranged downstream of an image forming apparatus body to perform postprocessing, such as stapling, on recording sheets or the like output from the image forming apparatus are widely known. Such sheet processing apparatuses have come to incorporate multiple functions recently. Sheet processing apparatuses capable of making booklets by not only side stitching but also by saddle stitching have also become common. Some type of such sheet processing apparatuses that make booklets by saddle stitching further trims an edge(s) of a stapled booklet using a trimmer to increase quality of the finished products.
In such a trimmer, a booklet to be trimmed is typically conveyed by a conveying unit such as a belt and positioned by being abutted against an abutting stopper that is located at a position appropriate for a size of the booklet, a trim size, and/or the like. Thereafter, a trimming unit trims an edge of the booklet that is fixed by being pressed by a pressing unit. As a result, the stapled booklet has a flush finished edge.
However, in the conventional trimmer, the conveying unit such as a belt rotates in a state where the booklet is in contact with the positioning stopper, causing a surface sheet of the booklet to be swelled out. When the booklet is fixed in this state and trimmed, the edge of the trimmed booklet becomes uneven. Furthermore, when the pressing unit presses the booklet in a manner that the pressing unit flattens the swelling of the booklet, flexure develops on a surface side or a spine side of the booklet during the pressing, resulting in that the booklet is moved forward or backward and pressed in a misaligned state. Accordingly, trimming the booklet in such a pressed state can undesirably make the edge of the trimmed booklet uneven.
An example of devices that perform trimming without developing such flexure is disclosed in Japanese Patent Application Laid-open No. 2004-196494. In this device, a pressing force is gradually applied from a spine side (e.g., an end portion on a folded side) of the sheets using a plate, a roller, or the like to a folded sheet stack in a manner to move the sheets toward their end surfaces before the sheets are fixed by being pressed by a stack pressing member, thereby preventing flexure between a position where the sheet stack is pressed and the spine side of the sheets.
Hereinafter, “pressing” means generating a pressure by pressing something; “pressing force” is a pressure generated by pressing, or, put another way, a force exerted to press something. “When being pressed” refers to a situation where an operation of pressing a subject is performed, or, put another way, when the subject is pressed. “Abutting” means that something comes into contact with an object and this abutted (contact) condition is maintained.
Disclosed in Japanese Patent Application Laid-open No. 2004-196494 is a technique for correcting flexure and/or misalignment of the booklet before the booklet is pressed. This technique makes it possible to rectify flexure and/or misalignment of the booklet. This technique also prevents development of flexure in a booklet when the booklet is pressed, and is presumably effective in rectifying flexure in the booklet. However, the booklet can move toward its end surface when a pressing force is applied to the booklet. Trimming the booklet in the moved state can undesirably cause the booklet to have a slanted trimmed face.
The reason why the booklet moves toward its end surface is described more specifically in greater depth below. FIGS. 20 and 21 are schematic explanatory diagrams illustrating a disadvantage pertaining to the conventional technique. FIG. 20 illustrates an example where a booklet BT is pressed by a guide member 321 which is on a top side. Referring to FIG. 20, in this example, the guide member 321 arranged in an upper portion of a booklet conveying unit presses the booklet BT toward a first conveying belt 310 arranged in a lower portion of the booklet conveying unit so that the first conveying belt 310 conveys the booklet BT.
In this configuration, a conveying pressure is applied to the booklet BT by lowering the guide member 321. However, a frictional force between the first conveying belt 310 and the booklet BT is greater than a friction force between the guide member 321 and the booklet BT (if the former is not greater than the latter, conveyance cannot be performed). Accordingly, when alignment is performed by conveying the booklet BT into abutment on a positioning stopper 317, the first conveying belt 310 rotates even after the booklet BT has abutted on the positioning stopper 317. As a result, a torque is generated in a direction indicated by arrow D11, causing an upper side of the booklet BT to move relative to a lower side of the booklet BT as indicated by arrow D12. Consequently, the fore edge portion of the booklet BT becomes uneven. After the booklet BT is positioned, misalignment of the booklet BT similar to that that occurs during the alignment of the booklet BT can also occur when the booklet is fixed by being pressed in a situation where coefficients of friction of the guide member 321 and the first conveying belt 310, which are guide members on the top side and the bottom side of the booklet BT, differ from each other.
After the booklet BT is positioned by being abutted to the positioning stopper 317, misalignment of the booklet BT similar to that that occurs during the alignment of the booklet BT can also occur when the guide member 321 is lowered to fix the booklet BT in a situation where coefficients of friction between upper and lower members which are brought into contact with the booklet BT differ from each other.
FIG. 21 illustrates an example where the booklet BT is pressed by a second conveying belt 312, which is on the top side. Referring to FIG. 21, in this example, the second conveying belt 312 arranged in the upper portion of the booklet conveying unit is moved toward the first conveying belt 310 arranged in the lower portion of the booklet conveying unit so that the first and second conveying belts 310 and 312 convey the booklet BT sandwiched therebetween.
When such a configuration as described above is employed, the first and second conveying belts 310 and 312 rotate in a state where the booklet BT abuts on the positioning stopper 317 while alignment is performed by causing the booklet BT to abut on the positioning stopper 317. Accordingly, a conveying force acts on a surface sheet of the booklet BT, thereby undesirably developing flexure BT10 near a spine of the booklet BT. After trimming of the booklet BT fixed in this state, the flexure of the surface sheet is straightened. Accordingly, the finished booklet BT undesirably has an uneven fore edge portion.
There is a need of a booklet pressing device that fixes a booklet by pressing the booklet without causing a surface sheet to be misaligned.